Seismicity and velocity structure in the vicinity of repeating slow slip earthquakes, northern Hikurangi subduction zone, New Zealand

Abstract

Using an 11-month (April 2012 to March 2013) deployment of Ocean Bottom Seismographs along the northern Hikurangi subduction zone, New Zealand, we located 2005 earthquakes (M 0.7-4.5) and obtained detailed 3-D Vp and Vs models for a region where a spectrum of subduction fault slip behavior is observed. Our analysis over a 120 × 110 km2 onshore-offshore area reveals high Vp/Vs (>1.8) in the subducting 10 km thick Hikurangi Plateau oceanic crust below an area of repeating slow slip events (SSEs), that are assumed to occur on the plate megathrust. Low Vp/Vs is mapped in a 15 km wide domain along the coastline, offshore Gisborne, marked by low Qs/Qp. Seismicity is absent from the core of high Vp/Vs anomalies (∼2.0) and within the band with lower Vp/Vs, but earthquakes are distributed vertically and are widespread in adjacent high Vp/Vs (∼1.85-1.95) regions along the trend of the coastline. Down-dip and along-strike variation of the Vp/Vs structure of the subducting Hikurangi Plateau could be the result of dehydration process caused by reactivated bending-related faults. Zones of vertical extensional fault–fracture in the subducting crust may act as conduits for the discharge of overpressured fluid and episodic hydrofracturing. During our deployment a large SSE (Mw6.8) occurred in February 2013. In response, seismicity became active in central Hawke Bay and at the down-dip edge of a previously mapped high-amplitude reflectivity zone. Microseismicity appeared synchronous with the slip propagation of the SSEs and may have been initiated by the slip. Our results hold implications for understanding of the physical environments conducive to slow slip generation and represent an important baseline for future seismicity work in the region.